Traffic on the Internet keeps growing, due in large part to the increasing demand from mobile devices, streaming media services, cloud computing, and big data analysis. Silicon photonics is promising for providing high-speed, low energy consumption and low cost next generation data communication systems. The last decade has witnessed dramatic improvement and maturity of silicon photonics devices. High quality hybrid integrated lasers with sub-MHz linewidth, modulators and photodetectors supporting 40 Gb/s or higher data rates have all been demonstrated. See T. Creazzo, E. Marchena, S. B. Krasulick, P. K.-L. Yu, D. Van Orden, J. Y. Spann, C. C. Blivin, L. He, H. Cai, J. M. Dallesasse, R. J. Stone, and A. Mizrahi, “Integrated tunable CMOS laser,” Opt. Express 21(23), 28048-28053 (2013); S. Yang, Y. Zhang, D. W. Grund, G. A. Ejzak, Y. Liu, A. Novack, D. Prather, A. E.-J. Lim, G.-Q. Lo, T. Baehr-Jones, and M. Hochberg, “A single adiabatic microring-based laser in 220 nm silicon-on-insulator,” Opt. Express 22(1), 1172-1180 (2014); D. J. Thomson, F. Y. Gardes, J.-M. Fedeli, S. Zlatanovic, Y. Hu, B. P.-P. Kuo, E. Myslivets, N. Alic, S. Radic, G. Z. Mashanovich, and G. T. Reed, “50-Gb/s silicon optical modulator,” IEEE Photon. Technol. Lett. 24(4), 234-236 (2012); T. Baba, S. Akiyama, M. Imai, N. Hirayama, H. Takahashi, Y. Noguchi, T. Horikawa, and T. Usuki, “50-Gb/s ring-resonator-based silicon modulator,” Opt. Express 21(10), 11869-11876 (2013); C. T. DeRose, D. C. Trotter, W. A. Zortman, A. L. Starbuck, M. Fisher, M. R. Watts, and P. S. Davids, “Ultra compact 45 GHz CMOS compatible Germanium waveguide photodiode with low dark current,” Opt. Express 19(25), 24897-24904 (2011); and L. Vivien, A. Polzer, D. Marris-Morini, J. Osmond, J. M. Hartmann, P. Crozat, E. Cassan, C. Kopp, H. Zimmermann, and J. M. Fédéli, “Zero-bias 40 Gbit/s germanium waveguide photodetector on silicon,” Opt. Express 20(2), 1096-1101 (2012).
Transceivers and switch fabrics monolithically integrated with electronics have been reported. See B. Analui, D. Guckenberger, D. Kucharski, and A. Narasimha, “A fully integrated 20-Gb/s optoelectronic transceiver implemented in a standard 0.13-μm CMOS SOI technology,” IEEE J. Solid-State Circuits 41(12), 2945-2955 (2006); J. F. Buckwalter, X. Zheng, G. Li, K. Raj, and A. V. Krishnamoorthy, “A monolithic 25-Gb/s transceiver with photonic ring modulators and Ge detectors in a 130-nm CMOS SOI process,” IEEE J. Solid-State Circuits 47(6), 1309-1322 (2012); and B. G. Lee, A. V. Rylyakov, W. M. J. Green, S. Assefa, C. W. Baks, R. Rimolo-Donadio, D. M. Kuchta, M. H. Khater, T. Barwicz, C. Reinholm, E. Kiewra, S. M. Shank, C. L. Schow, and Y. A. Vlasov, “Monolithic silicon integration of scaled photonic switch fabrics, CMOS logic, and device driver circuits,” J. Lightw. Technol. 32(4), 743-751 (2014). Coherent long-haul communication at 112 Gb/s was also demonstrated. See P. Dong, X. Liu, S. Chandrasekhar, L. L. Buhl, R. Aroca, Y. Baeyens, and Y.-K. Chen, “Monolithic silicon photonic integrated circuits for compact 100+Gb/s coherent optical receivers and transmitters,” IEEE J. Sel. Topics Quantum Electron. 20(4), 6100108 (2014). Foundry services open access of advanced fabrication nodes to academic labs and startups, which would further speed up research and development of photonic integration on silicon. Se for example M. Hochberg and T. Baehr-Jones, “Towards fabless silicon photonics,” Nat. Photonics 4, 492-494 (2010); and A. E.-J. Lim, J. Song, Q. Fang, C. Li, X. Tu, N. Duan, K. K. Chen, R. P.-C. Tern, and T.-Y. Liow, “Review of silicon photonics foundry efforts,” IEEE J. Sel. Topics Quantum Electron. 20(4), 8300112 (2011).
One bottleneck that emerges during the design of silicon photonics based data links is the constraint on link power budget. A typical link power budget is around 9 dB. For example the IEEE 802.3 40GBASE-LR4 protocol has 6.7 dB allocated for channel insertion loss and 2.3 dB for penalties. Due to the large mode mismatch of glass fibers and submicron silicon waveguides, on-and-off chip coupling loss is usually quite high. The losses can exceed 1 dB in a mature commercial process. See A. Mekis, S. Gloeckner, G. Masini, A. Narasimha, T. Pinguet, S. Sahni, and P. De Dobbelaere, “A grating-coupler-enabled CMOS photonics platform,” IEEE J. Sel. Topics Quantum Electron. 17(3), 597-608 (2011). On-chip devices tend to be lossy as well. For example, insertion losses of state of the art silicon modulators are more than 5 dB. In some cases, device insertion loss could be significantly reduced by design optimization, such as the y-junction, the waveguide crossing and by grating couplers. See Y. Zhang, S. Yang, A. E.-J. Lim, G.-Q. Lo, C. Galland, T. Baehr-Jones, and M. Hochberg, “A compact and low loss Y-junction for submicron silicon waveguide,” Opt. Express 21(1), 1310-1316 (2013); Y. Ma, Y. Zhang, S. Yang, A. Novack, R. Ding, A. E.-J. Lim, G.-Q. Lo, T. Baehr-Jones, and M. Hochberg, “Ultralow loss single layer submicron silicon waveguide crossing for SOI optical interconnect,” Opt. Express 21(24), 29374-29382 (2013); and W. S. Zaoui, A. Kunze, W. Vogel, M. Berroth, J. Butschke, F. Letzkus, and J. Burghartz, “Bridging the gap between optical fibers and silicon photonic integrated circuits,” Opt. Express 22(2), 1277-1286 (2014). However, in other cases, insertion loss and device efficiency are orthogonal, for example, higher doping results in higher modulation efficiency, but leads to high insertion loss at the same time.
A photodetector with high responsivity will compensate the channel insertion loss, and help fulfill the required link power budget. Germanium can be epitaxially grown on silicon and is the preferred absorber material for its CMOS compatibility. Although metal-semiconductor-metal (MSM) and avalanche photodetector (APD) could provide high responsivity by photoconductive gain and avalanche multiplication, the benefit comes at the price of high dark current and (or) high bias voltage. Waveguide coupled p-i-n detectors attract extensive attention due to their high bandwidth, good responsivity and low dark current. Ge-on-Si detectors with lateral and vertical p-i-n junction configuration are illustrated in FIG. 1A and FIG. 1B. Attractive Ge-on-Si detector performances have been reported, with responsivity typically about 0.8 A/W and bandwidth high enough for 40 Gb/s operation. See for example T. Yin, R. Cohen, M. M. Morse, G. Sarid, Y. Chetrit, D. Rubin, and M. J. Paniccia, “31 GHz Ge n-i-p waveguide photodetectors on silicon-on-insulator substrate,” Opt. Express 15(21), 13965-13971 (2007); and A. Novack, M. Gould, Y. Yang, Z. Xuan, M. Streshinsky, Y. Liu, G. Capellini, A. E.-J. Lim, G.-Q. Lo, T. Baehr-Jones, and M. Hochberg, “Germanium photodetector with 60 GHz bandwidth using inductive gain peaking,” Opt. Express 21(23), 28387-28393 (2013) as well as some of the previously mentioned articles.
As shown in FIG. 1A and FIG. 1B, both types of device require heavily doped germanium to form the junction and direct contact of germanium to metal via plugs. Although the first transistor was demonstrated using germanium, silicon quickly took over and became the overwhelmingly dominating substrate material. Germanium processing has recently attracted attention because of interest in germanium and silicon-germanium transistors. See S. Brotzmann, and H. Bracht, “Intrinsic and extrinsic diffusion of phosphorus, arsenic, and antimony in germanium,” J. Appl. Phys. 103, 033508 (2008), A. Claverie, S. Koffel, N. Cherkashin, G. Benassayag, and P. Scheiblin, “Amorphization, recrystallization and end of range defects in germanium,” Thin Solid Films 518(9), 2307-2313 (2010); and H. Bracht, S. Schneider, and R. Kube, “Diffusion and doping issues in germanium,” Microelectron. Eng. 88(4), 452-457 (2011). Germanium is much less well understood and characterized as compared to silicon. While silicon modulators have been optimized for efficiency (see Y. Liu, S. Dunham, T. Baehr-Jones, A. E.-J. Lim, G.-Q. Lo, and M. Hochberg, “Ultra-responsive phase shifters for depletion mode silicon modulators,” J. Lightwave Technol. 31(23), 3787-3793 (2013)), similar TCAD models are still not seen for germanium detectors. Poly silicon was sometimes deposited on top of germanium to reduce contact resistivity and leakage current. See for example, C.-K. Tseng, W.-T. Chen, K.-H. Chen, H.-D. Liu, Y. Kang, N. Na, and M.-C. M. Lee, “A self-assembled microbonded germanium/silicon heterojunction photodiode for 25 Gb/s high-speed optical interconnects,” Sci. Rep. 3, 3225 (2013); and K. Takeda, T. Hiraki, T. Tsuchizawa, H. Nishi, R. Kou, H. Fukuda, T. Yamamoto, Y. Ishikawa, K. Wada, and K. Yamada, “Contributions of Franz—Keldysh and avalanche effects to responsivity of a germanium waveguide photodiode in the L-band,” IEEE J. Sel. Topics Quantum Electron. 20(4), 3800507 (2014).
There is a need for improved designs and structures for photodetectors made using germanium.